A Monoborylated Multiresonance Thermally Activated Delayed Fluorescence Emitter Enabling Efficient and Ultranarrowband Red Electroluminescence

The pursuit of high-efficiency narrowband red emitters for next-generation displays is compounded by the challenge of simultaneously achieving high synthetic yield, narrow spectral line width, and high exciton utilization. Here, we break this trade-off with a high-yield monoborylated multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitter, α-NAICZ-BN, which incorporates key structural motifs from distinct narrowband systems. Our molecular design simultaneously suppresses midfrequency vibrational broadening and enhances reverse intersystem crossing, enabling efficient red emission at 605 nm in solution with a record-narrow 26 nm full-width at half-maximum (fwhm) among monoborylated red MR-TADF emitters. The corresponding organic light-emitting diode achieves narrowband red electroluminescence at 617 nm with CIE coordinates of (0.67, 0.33), while attaining a maximum external quantum efficiency of 29.5% and ultrahigh power efficiency of 53.1 lm W^-1 with negligible efficiency roll-off. This work establishes a new design paradigm for high-performance red MR-TADF materials, demonstrating how single-boron molecular frameworks can achieve exceptional color purity and device efficiency simultaneously.